This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. Natures approach towards utilizing molecular hydrogen as an energy source is based on an own set of catalysts. Two of them which are termed as [FeFe] and [NiFe] hydrogenases contain binuclear metal cores as catalytic sites, whereas the third one termed [Fe] hydrogenase is mononuclear in origin. Though principal structural features of these enzymes have been obtained from protein crystallographic investigations important details of the catalytic mechanisms associated with biological hydrogen production or consumption are still not known. This lack of information is due to a particular problem: X-ray crystallography is practically blind for the substrate of these enzymes, e.g. for hydrogen atoms if they are coordinated to metal atoms or situated in their direct neighborship. Another lack of urgently needed information is associated with the [NiFe] hydrogenases which are isolated in oxidized unready stages. In the case of the Ni-B stage, a hydroxo or oxo ligand has been identified as a third bridge connecting Ni and Fe, but the exact bridging situation within the enzyme adopting the Ni-A stage is still the subject of a controversial debate due to oxidation damage of the enzyme. This damage results in a superposition of differently modified species, one of which is proposed as a hydroperoxo or peroxo bridge system which cannot be resolved unambiguously.